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We assume that Bob knows B and Enc(K1, ABC) but doesn't know K1, A or C. Then the only information Bob can learn about ABC is the length of AC (not even the length of A and C individually). If Bob knows Enc(K1, M), Bob can't even tell whether B is a substring of M. This is assuming that Enc is any sensible symmetric cipher, including AES-CBC (with a random ...

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